Preparation And Properties Of The Solid Oxide Fuel Cell Electrolyte Materials

Solid oxide fuel cell (SOFC) is an all-solid-state energy device which could convert directly the chemical energy of fuels and oxidants into electrical energy. At present the most electrolyte materials used in the SOFC system are yttrium stable zirconia (YSZ). YSZ requires a high operating temperature (900-1000℃) to get high enough ionic conductivity, and this will cause problems such as the chemical reaction of battery components, electrode sintering and thermal expansion coefficient mismatch. Ce0.8Sm0.2O1.9-based electrolytes with a fluorite structure and La_9.33Si₆O₂₆-based electrolytes with an apatite structure were studied in this thesis. Influences of doping and composite technology on the conductive properties of these two electrolyte materials were studied. The research work comprises the following aspects.(1) Chemical co-precipitation method was employed to synthesize the Ce_0.8Sm_0.2-xY_xO_1.9(x=0,0.05,0.1,0.15) powers and the conventional sintering technique was utilized to prepare the related ceramics. The structure and electrical properties of the electrolyte materials were characterized by XRD, SEM and ac impedance technology, respectively. The influence of Y³⁺ contents on the electrical properties of Ce_0.8Sm_0.2-xY_xO_1.9 ceramics was studied. With Y³⁺ content increases, electrical conductivities of electrolytes increase first and then decrease and reach a maximum value when x=0.1, the total conductivity is 7.94×10-3 S·cm^-1 at 700℃. The conductive mechanism was discussed preliminary.(2) Chemical co-precipitation method was employed to synthesize the La9.3³⁺xSi6O26+1.5x (x=0,0.2,0.4) powers and the conventional sintering technique was utilized to prepare the related ceramics. The structure and electrical properties of the electrolyte materials were characterized by XRD and ac impedance technology. The influence of La³⁺ contents on the electrical properties of La9.3³⁺xSi6O26+1.5x ceramics was studied. Addition of excessive La³⁺ could enhance obviously electrical conductivities of electrolytes. From x=0 to x=0.4, conductivities increase from 2.5×10-4Scm^-1to 1.48×10-3S·cm^-1 at 700℃, about 6 times improved. The conductive mechanism was discussed with Jonscher experience formula, and long-range conduction mechanism was regarded as the dominant one.(3) Ce0.8Sm0.201.9-La9.33Si6026.6 composite oxygen ion conductors were prepared by composite technology. The second phase was introduced via chemical co-precipitation, solvothermal and ball milling mixing. The composite electrolyte materials with different composite amounts were prepared. The conventional sintering technique was utilized to prepare the related ceramics. The phase and electrical properties of the electrolyte materials were characterized by XRD, SEM, EDS and ac impedance technology. The results show that composite electrolytes consist of Ce0.8Sm0.2O19 and La9.33Si6026-Interfaces between phases play important role in total conductivities. The influences of quantity and introducing way of the second phase on the composites conductive properties were studied.(4) Ce0.8Sm0.1Y0.1O1.9-La9.73Si6026 composite electrolyte was prepared by two-step co-precipitation method. The electrical properties were characterized by ac impedance technology. The results showed that phase purity and solid solution reaction between interfaces have important influences on the performances of composite electrolytes.